The interaction with the lower ionosphere of rapidly varying electromagnetic pulses (EMPs) produced by lightning discharges is studied. The nonlinear heating, ionization and optical emission production are modeled using the Monte Carlo technique, which allows for consideration of realistic lightning EMPs with a few &mgr;s rise times. Results indicate that the electron distribution function is highly anisotropic during the first few &mgr;s of the interaction, but subsequently develops into a near-isotropic quasi-stationary state. The peak optical emissions intensities are found to be highly dependent on the EMP waveform, while the altitude range at which the emissions occur is relatively independent of pulse shape. Results of the particle simulation are used to assess the range of applicability of the quasi-stationary models [Taranenko et al., 1993; Inan et al., 1996>. ¿ American Geophysical Union 1996